The field is two-stroke cycle, opposed-piston engines. More specifically the application relates to a turbocharged, opposed-piston engine in which a supercharger provides boost pressure under start-up conditions and drives EGR during normal operating conditions.
A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. One example of a two-stroke cycle engine is an opposed-piston engine in which two pistons are disposed in opposition in the bore of a cylinder for reciprocating movement in opposing directions. The cylinder has longitudinally-spaced inlet and exhaust ports formed in the cylinder sidewall near respective ends of the cylinder. Each of the opposed pistons controls one of the ports, opening the port as it moves to a bottom center (BC) location, and closing the port as it moves from BC toward a top center (TC) location. One of the ports provides passage for the products of combustion out of the bore, the other serves to admit charge air into the bore; these are respectively termed the “exhaust” and “intake” ports. In a uniflow-scavenged opposed-piston engine, charge air enters a cylinder through its intake port as exhaust gas flows out of its exhaust port, thus gas flows through the cylinder in a single direction (“uniflow”)—from intake port to exhaust port.
Air and exhaust products flow through the cylinder via an air handling system. Fuel is delivered by injection from a fuel delivery system. As the engine cycles, a control mechanization governs combustion by operating the air handling and fuel delivery systems in response to engine operating conditions. The air handling system may be equipped with an exhaust gas recirculation (“EGR”) system to reduce undesirable compounds produced by combustion.
In an opposed-piston engine, moving fresh air into the intake manifold and exhausting spent gases out of the engine require pumping work. In typical opposed-piston engine air handling systems the pumping work is done by one or more pumps, such as a supercharger or blower, either of which takes its power from the engine crank. If a turbocharger is included in the engine air handling system, it uses some of the exhaust energy to increase the intake air density to provide a higher mass of trapped air in the cylinder and requires pumping energy only from the exhaust gasses.
The pumping device that drives air from the intake to the exhaust drives the scavenging process, which is critical to ensuring effective combustion, increasing the engine's indicated thermal efficiency, and extending the lives of engine components such as pistons, rings, and cylinders. This pumping work also drives the EGR system by creating a pressure difference across the EGR channel. However, in some operating conditions like low load situations, the EGR requirements for reducing NOx may be much higher than the trapped air requirements for good combustion. When this happens, the trapped air mass ends up being higher than needed, which can result in higher NOx production as well as dilution of recirculated exhaust gas by oxygen molecules, making EGR less potent for reducing NOx. This will increase the required exhaust recirculation rate to meet NOx standards and therefore will result in higher pumping losses. Therefore, total pumping work of the air handling system would be reduced by separating the in-cylinder air requirements for the scavenging process from the EGR requirements.
Two-stroke engines with large displacements and high power ratings are equipped with air handling systems capable of pumping large amounts of air. Consider, for example a two-stroke cycle, opposed-piston engine having a displacement of 15 L (or higher) that is rated at 500+ hp. The mass air flow required for operation of such an engine necessitates large pumping devices. In some large, turbocharged opposed-piston engines, turbo device size benefits engine operation. In this regard, large turbochargers have large volumes compared to their surface areas, which result in very efficient pumping operation. For example, large turbochargers may exhibit compressor efficiencies in excess of 80% and turbine efficiencies that approach 80%. During typical engine operating conditions, compressor-out pressures are higher than the turbine inlet pressures because of higher efficiencies delivered by these devices, thereby creating a pressure difference between intake and exhaust to drive scavenging. In many instances, large engines that use large turbochargers typically require a supercharger or compressed air from an auxiliary device to deliver boost pressure for start-up. However once the engine is running with a sufficient amount of mass flow through it, the compressor provides sufficient boost. In these large engines an additional pumping device is usually needed for driving EGR because the exhaust pressures are lower than the turbocharger-driven compressor output pressure.
In order to limit engine size, complexity, and cost, it is desirable to reduce the number of auxiliary pumping devices in the air handling system of a turbocharged opposed-piston engine. It is further desirable to arrange the reduced number of pumping devices so as to reduce pumping losses by separating mass air flow from EGR. Yet further benefit is realized if the reduced number of pumping devices is attached to, or integral with, the engine, which permits the engine to be used at various locations and for a variety of applications where a compressed air supply separate from the engine may not be available to provide boost for starting the engine.